gromgull · October 20, 2012 06:01 · Oct 20, 2012
diff --git a/unmatchedgroup.py b/unmatchedgroup.py
@@ -0,0 +1,28 @@
+import re
+
+def re_sub(pattern, replacement, string):
+	def _r(m):
+		# Now this is ugly.
+		# Python has a "feature" where unmatched groups return None
+		# then re.sub chokes on this.
+		# see http://bugs.python.org/issue1519638
+
+		# this works around and hooks into the internal of the re module...
+
+		# the match object is replaced with a wrapper that
+		# returns "" instead of None for unmatched groups
+
+		class _m():
+			def __init__(self, m):
+				self.m=m
+				self.string=m.string
+			def group(self, n):
+				return m.group(n) or ""
+
+		return re._expand(pattern, _m(m), replacement)
+
+	return re.sub(pattern, _r, string)
+
+print re_sub('(ab)|(a)', r'(1:\1 2:\2)', 'abc')
+# prints '(1:ab 2:)c'
+